Xylene isomers find wide and varied application. They are especially valuable as intermediates in chemical processes. By way of example, para-xylene (PX) is a feedstock for terephthalic acid, which is used in the manufacture of polyester fibers and films, meta-xylene (MX) is used in the manufacture of dyes, and ortho-xylene (OX) is used as a feedstock for phthalic anhydride, which itself is used in the manufacture of plasticizers. PX is currently the most valuable of the xylene isomers and, although research related to obtaining (e.g., producing or purifying) PX is too voluminous to mention, there is still intensive research in the area.
There are many possible feeds currently used to obtain PX. The majority of para-xylene produced today comes from catalytic reforming, which involves dehydrogenation and dehydrocyclization of naphtha feedstocks. The effluent of the reforming process, known as reformate, is rich in aromatics, particularly benzene, toluene and mixed xylenes (BTX), and is used as feedstock to many aromatics plants. Processes exist to increase the yield of para-xylene over the equilibrium mixture in the reformate, including selective toluene disproportionation and selective methylation of benzene and/or toluene with methanol.
Recently, significant research has focused on finding alternative sources and methods for producing BTX and particularly para-xylene. For example, although steam cracking, or pyrolysis, is the preferred method of producing light olefins (ethylene, propylene, and butenes) from heavier hydrocarbon feedstocks, the process also generates a by-product termed pyrolysis gasoline, steam cracked naphtha (SCN) or pygas. Pygas is a complex mixture of C6 to C10+ hydrocarbons that is rich in aromatics, particularly benzene and toluene, but also contains C8, C9, and C10+ aromatics. Similarly, catalytic cracking, particularly fluid catalytic cracking (FCC), in addition to producing fuels and light olefins, generates a C6 to C10 aromatic rich stream which is similar to pygas and is generally known as cat naphtha. There is significant interest in developing methods of upgrading alternate sources, such as pygas and cat naphtha, to increase the yield of BTX and preferably para-xylene.
For example, U.S. Pat. No. 6,635,792 discloses a process for producing BTX and liquefied petroleum gas (LPG) from a hydrocarbon feedstock having boiling points of 30° C. to 250° C., such as reformate and pyrolysis gasoline. In the process, aromatic components in the hydrocarbon feedstock are converted to BTX-enriched components in the liquid phase through hydrodealkylation and/or transalkylation, and non-aromatic components are converted to LPG-enriched gaseous materials through hydrocracking. The process employs a catalyst comprising platinum/tin or platinum/lead on mordenite, zeolite beta or ZSM-5 and is said to have the advantage of avoiding the need of a solvent extraction step to remove aliphatic compounds from the hydrocarbon feedstock. U.S. Pat. Nos. 7,297,831 and 7,301,063 disclose similar processes.
U.S. Pat. No. 7,176,339 discloses a process for producing xylenes from reformate, which process comprises: (a) providing a reformate containing hydrogen, C1 to C5 hydrocarbons, C6 to C7 hydrocarbons comprising benzene, toluene or mixtures thereof, and C8+ hydrocarbons; (b) removing at least a portion of said hydrogen from said reformate to produce a product containing C6 to C7 hydrocarbons comprising benzene, toluene or mixtures thereof, and C8+ hydrocarbons; and (c) methylating at least a portion of the benzene, toluene, or mixtures thereof present in said product with a methylating agent under vapor phase conditions and in the presence of a catalyst effective for the methylation to produce a resulting product having a higher para-xylene content than the reformate, wherein the catalyst comprises a zeolite-bound-zeolite catalyst and/or a selectivated zeolite and the zeolite comprises ZSM-5. One of the problems alleged to be overcome by this process is the need for an expensive aromatics extraction step to separate C6 to C7 aromatics from the C6 to C7 aliphatics after removal of the hydrogen and C1 to C5 hydrocarbons. A similar process is disclosed in U.S. Pat. No. 7,629,498.
U.S. Pat. No. 7,563,358 discloses a process for producing BTX-enriched product from a hydrocarbon feed comprising: (a) C6+ non-aromatic cyclic hydrocarbons; (b) C8+ single-ring aromatic hydrocarbons having at least one alkyl group containing two or more carbon atoms; and (c) C9+ single-ring aromatic hydrocarbons having at least three methyl groups, by contacting the feed in the presence of hydrogen with a catalyst comprising at least one Group VIII metal and a large or intermediate pore molecular sieve having an alpha value, before incorporation of the Group VIII metal, from about 2 to less than 100 under conditions sufficient for (i) forming aromatic hydrocarbons from C6+ non-aromatic cyclic hydrocarbons; (ii) dealkylating C8+ single-ring aromatic hydrocarbons having at least one alkyl group containing two or more carbon atoms; (iii) transalkylating C9+ single-ring aromatic hydrocarbons having at least three methyl groups; and (iv) disproportionating toluene, to produce a product containing an increased amount of BTX compared to the feed. A preferred hydrocarbon feed is steam cracked naphtha.
U.S. Published Patent Application No. 2009/000988 discloses a process of manufacturing para-xylene, comprising: (a) contacting a pygas feedstock and methylating agent with a catalyst under reaction conditions to produce a product having para-xylene, wherein said product has higher para-xylene content than the para-xylene content of the pygas feedstock; and (f) separating said para-xylene from the product of the step (a), wherein said catalyst comprises a molecular sieve having a Diffusion Parameter for 2,2-dimethylbutane of about 0.1 to 15 sec−1 when measured at a temperature of 120° C. and a 2,2-dimethylbutane pressure of 8 kPa-a and said pygas comprises from about 1 wt % to about 65 wt % benzene and from about 5 wt % to 35 wt % toluene.
In U.S. Application Ser. No. 61/421,917 filed Dec. 10, 2010 (and U.S. Ser. No. 13/303,855), we have described a hydrocarbon upgrading process comprising (a) treating a first hydrocarbon stream in at least one of a steam cracker, catalytic cracker, coker, hydrocracker, reformer, and the like, under suitable conditions to produce a second stream comprising C6 to C10+ aromatic hydrocarbons; (b) dealkylating and/or transalkylating and/or cracking (D/T/C) the second stream by contact with a suitable catalyst under suitable reaction conditions to produce a third stream having an increased benzene and/or toluene content compared with the second stream and a light paraffin by-product; and (c) methylating at least a portion of the third stream with a methylating agent to selectively produce para-xylene. By integrating different upgrading steps, this process offers significant advantages in terms of higher petrochemical yields and lower energy consumption as compared with existing processes for enriching the BTX content of hydrocarbon streams.
Further investigation into the process described in U.S. Application Ser. No. 61/421,917 (and U.S. Ser. No. 13/303,855) has, however, shown that for hydrocarbon feeds containing large amounts of non-aromatic compounds, the process has challenges of high hydrogen consumption, production of low value products, such as LPG, and potential catalyst aging concerns. In addition, it has been found, contrary to the teaching of the prior art, such as U.S. Pat. Nos. 6,635,792 and 7,176,339, that reduction of the level of non-aromatics to the D/T/C step decreases the hydrogen consumption, reduces LPG made in favor of higher value C6+ raffinate production and reduces aging of the D/T/C catalyst. Surprisingly, introduction of an aliphatics removal step, even though costly, can actually reduce overall investment by decreasing the size of the D/T/C and methylation reactors.